This invention relates to the preparation of diaryl carbonates by carbonylation. More particularly, it relates to the employment of a simplified catalyst composition in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by a carbonylation reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction has required a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 5,231,210, 5,284,964 and 5,399,734 and copending, commonly owned application Ser. No. RD-25582!. The catalyst compositions described therein comprise a Group VIIIB metal (i.e., a metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum) or a complex thereof. They are used in combination with a bromide source, as illustrated by tetra-n-butylammonium bromide and hexaethylguanidinium bromide.
Other catalytic constituents are necessary in accordance with the above-listed patents and application. They include inorganic cocatalysts, typically complexes of cobalt(II) salts with organic compounds capable of forming complexes, especially pentadentate complexes, therewith. Illustrative organic compounds of this type are nitrogen-heterocyclic compounds including pyridines, bipyridines, terpyridines, quinolines, isoquinolines and biquinolines; aliphatic polyamines such as ethylenediamine and tetraalkylethylenediamines; crown ethers; aromatic or aliphatic amine ethers such as cryptanes; and Schiff bases. The especially preferred inorganic cocatalyst in many instances is a cobalt(II) complex with bis3-(salicylalamino)propyl!methylamine.
Organic cocatalysts are also necessary. They may include various terpyridine, phenanthroline, quinoline and isoquinoline compounds including 2,2':6',2"-terpyridine, 4-methylthio-2,2':6',2"-terpyridine and 2,2':6',2"-terpyridine N-oxide, 1,10-phenanthroline, 2,4,7,8-tetramethyl-1,10-phenanthroline, 4,7-diphenyl-1,10, phenanthroline and 3,4,7,8-tetramethy-1,10-phenanthroline. The terpyridines and especially 2,2':6',2"-terpyridine have generally been preferred.
It would be desirable to conduct the carbonylation reaction in the presence of a catalyst composition comprising fewer constituents, and especially one which does not require the presence of relatively expensive cobalt complexes and terpyridines. The present invention involves the use of such a catalyst composition.